1. Field of the Invention
The invention relates to a method for operation of an arrangement having at least one superconducting cable, which is surrounded by a cryostat which consists of two metallic tubes, which are arranged concentrically with respect to one another and enclose vacuum insulation between them, and which cryostat surrounds not only the cable but also a cavity for a pressurized coolant to pass through.
2. Description of the Related Art
A method such as this is disclosed in WO 03/052775 A.
In modern technology, a superconducting cable has electrical conductors composed of a composite material which contains ceramic material that changes to the superconducting state at sufficiently low temperatures. The electrical direct-current resistance of a correspondingly constructed conductor is zero, if the cooling is adequate, provided that a specific current level is not exceeded. By way of example, suitable ceramic materials are materials doped with rare earths, which have become known by the name ReBCO (rare-earth barium-copper oxide), in particular including YBCO (yttrium-barium-copper oxide). Another of these superconducting materials is, for example, BSCCO (bismuth-strontium-calcium-copper oxide). Sufficiently low temperatures to change a material such as this to the superconducting state are, for example, between 67 K and 110 K. Suitable coolants are, for example, nitrogen, helium, neon and hydrogen, or mixtures of these substances.
When carrying out the method described initially and during operation of the associated arrangement, the liquid coolant is forced into the cryostat at a predetermined pressure by means of the pump, is passed through the cryostat while maintaining the pressure, and is passed out to be cooled down again at the end of the predetermined length. For adequate cooling of the at least one superconducting conductor in the cable located in the cryostat, a considerable amount of coolant must be lowered to the required low temperature, and must be kept at this temperature in the reservoir area, and if possible in the cryostat as well. If, in the course of a transmission path, the cryostat is damaged, in particular with a leak occurring in it, for example as a result of external mechanical damage to the cryostat, a large amount of coolant can emerge from the cryostat before, for example, a leak is found. This is not only expensive but is also hazardous to the area around the arrangement, particularly if liquid nitrogen is used as the coolant. This is also true when a leak actually occurs in the supply line of the coolant to the cryostat.
U.S. Pat. No. 3,946,141 A describes a method for cooling an electrical cable which is equipped with superconducting cables, in which the conductors are arranged in the form of a coaxial cable. The cable consists of two parts which merge into one another in the central area of the cable and are equipped with end closures at each of their ends. In its central area, connecting the two parts, a coolant, such as helium, is passed from a cooling station to the cable by means of a pump and is fed from there into the two parts of the cable in the opposite flow direction. The coolant is passed out at each of the end closures of the cable, and is fed back into the cooling station.
U.S. Pat. No. 3,950,606 discloses a method for cooling a superconducting cable whose conductors are likewise arranged in the form of a coaxial cable, having a tubular inner conductor and an outer conductor which concentrically surrounds it, with a dielectric inbetween them. A tube consisting of two tube elements separated by a cavity surrounds the outer conductor and a coolant, for example helium, is passed through it, and is fed back to the refrigeration unit by means of a separate line. The coolant is also passed through the tubular inner conductor in the same sense.
WO 03/052775 A1, cited initially, describes a superconducting cable with a cold dielectric. This is accommodated in a cryostat which consists of two tubes which are separated by vacuum insulation and run concentrically with respect to one another. This document deals essentially with the connection through the cryostat when at least two lengths of an arrangement such as this are intended to be connected to one another. The document likewise provides no details of measures to be taken in the event of damage to the cryostat, when a coolant is fed into the free space in the cryostat.